In the nuclear sector, elemental alkali metals are being used as coolant in the primary circuit of fast breeder reactors. Classical safety is a very critical issue for the treatment and conditioning of these metals, due to their chemical reactivity, and considering the quite severe acceptance criteria of the final waste. The SCK•CEN has conceived and patented a new and safe process to meet safety requirements and the compatibility with the further conditioning of the waste into an acceptable form. In this process, molten alkali metal reacts with a mixture of oxygen and carbon dioxide, in a fluidized bed reactor containing calibrated sand particles, to yield the metal oxide and carbonate, while avoiding the formation of peroxide. No hydrogen is formed, while the turbulent conditions guarantee a complete conversion of the metal and excellent heat transfer capabilities, eliminating explosion risks and reducing greatly fire risks. Depending on the level of radioactive contamination, the mixture of sand and carbonates may be considered as the final waste, or may be further conditioned as glass.